Kinetic modeling of lactose hydrolysis with an immobilized beta-galactosidase from Kluyveromyces fragilis

The kinetic model of the hydrolysis of lactose with a P-galactosidase from Kluyveromyces fragilis immobilized on a commercial silica-alumina (KA-3, fr om Sudchemie) has been determined. A wide experimental range of the main va riables has been employed: temperature, concentrations of substrate, and pr oducts and concentration of enzyme. The runs were performed in a complex bu ffer with the salt composition of milk. The effect of pH and temperature on the stability and the activity of the enzyme have been studied. The optimu m pH for the enzyme activity was, approximately, seven. The immobilized enz yme was more stable than the free one at acidic pH, but more instable at ba sic pH. The maximum temperature used for the hydrolysis runs performed to s elect the kinetic model was 40 degrees C, so inactivation of the enzyme dur ing the kinetic runs has been avoided. Agitation, concentration of enzyme i n the solid and particle size were selected to ensure that the overall rate was that of the chemical reaction. Eleven kinetic models were proposed to fit experimental data, from first order to more complex ones, such as those taking into account inhibition by one of the compounds involved in the hyd rolysis reaction. Applying statistical and physical criteria, a Michaelis-M enten model with a competitive inhibition by galactose has been selected. T he model is able to fit the experimental data correctly in the wide experim ental range studied. Finally, the model obtained is compared to the one sel ected in a previous work for the hydrolysis of lactose with the free enzyme . (C) 2000 Elsevier Science Inc. All rights reserved.